doc: v20.2.3 tentacle release notes

doc: v20.2.3 tentacle release notes

Adds the v20.2.3 tentacle release notes: Notable Changes by component
and the full Changelog, following the same structure as
ceph/ceph#69886.

Notable Changes are curated from the merged-PR changelog per
component (MDS, OSD, RGW, RADOS/librados/neorados, Dashboard,
ceph-volume, NVMe-oF, RBD), skipping test-only, CI, and
internal-refactor entries.

Addresses review feedback:
- afreen23: added a Dashboard bullet for the RGW Roles tab removal /
  move under RGW Accounts.
- idryomov/caroav: added an NVMe-oF bullet on the new mgr module and
  its automatic ".nvmeof" metadata-pool creation.
- batrick: release date corrected to August 5, 2026 in both
  tentacle.rst and releases.yml.
- idryomov: moved the two librbd bullets out of "RADOS / librados /
  neorados" (not RADOS-core changes) into a renamed "RBD" section
  (was "rbd-mirror").
- caroav: reworded the two terse NVMe-oF bullets (delayed failback,
  DELETING-state beacon handling) per her clarifications.

Release date: August 5, 2026.

Signed-off-by: Yuri Weinstein <yweinste@redhat.com>